In optical information processing systems or optical communication systems, the control of optical signals such as optical modulation for carrying signals on light or optical arithmetic operation are carried out.
In a conventional optical system, the control of optical signals is carried out by an electric control signal, in which a semiconductor laser as a light source is modulated directly by controlling a bias current of the semiconductor laser. In other case, the refractive index or the optical absorption coefficient of a semiconductor material or a dielectric material which composes a waveguide is changed by applying a voltage.
In the optical system controlled by an electric control signal, the operation speed of the optical system is limited by two factors, one of which is the operation speed of electrically operating devices which compose the optical system, and the other is mismatching of speeds between the electric control signal and a signal light to be controlled, so that it is very difficult to increase the operation speed of the optical system, or it is difficult to shorten the operation time shorter than the order of nano seconds.
Recently, an optical system controlled by a control light (opto-optic control system) has been developed. In the optical system controlled by a control light, there is no limitation in operation speed due to the non-existence of CR time constant, so that the operation speed is expected to increase considerably.
One type of an optical device for an optical system controlled by a control light is a nonlinear optical device which comprises a waveguide consisting of an optical nonlinear semiconductor material having a property of a nonlinear refractive index.
In operation, a signal light to be controlled is supplied to the waveguide of the nonlinear optical device. The signal light passes through the waveguide without any modulation when a control light is not supplied to the waveguide. When the control light having a predetermined wavelength is supplied to the waveguide, electrons in the valence band in the semiconductor composing the waveguide are excited into the conduction band to change the refractive index of the waveguide (band filling effects). As a result, a phase shift of the signal light occurs.
Therefore, the operation characteristic of the nonlinear optical device largely depends on the optical nonlinearity (the nonlinear index of refraction) of the semiconductor composing the waveguide. The optical nonlinearity is represented by the third order nonlinear optical susceptibility .chi..sup.(3) due to optical excitation of electrons from the valence band into the conduction band in a semiconductor.
The third order nonlinear optical susceptibility .chi..sup.(3) is described by D.A.B. Mirror et al on pages 221 to 226 of "Optics Communication, vol. 35-2, 1980". In the reference, the third order nonlinear optical susceptibility .chi..sup.(3) is given by the formula (11), however, there is an obvious misprint therein. Therefore, the third order nonlinear optical susceptibility .chi..sup.(3) will be recalculated by the substantially same method as follows: ##EQU1##
Where, e is a charge of an electron, .pi.=.pi./(2 .pi.), h is Plunck's constant, P is a dipole moment between the conduction and valence bands, .omega..sub.g =E.sub.g /.pi., E.sub.g is a band gap, .mu. is a reduced mass of electron and hole effective masses, and T.sub.1 and T.sub.2 are longitudinal and transverse relaxation times, respectively. The analysis is extended to cover the case in which a frequency .omega..sub.p of a pump light (control light) and a frequency .omega..sub.s of a signal light are different.
In the calculation shown above, only the real part of .chi..sup.(3) is calculated, because only the nonlinear refractive index change should be considered. The change of the refractive index n of the semiconductor is represented as follows: EQU n=n.sub.0 +n.sub.2 I (2)
Where, n.sub.0 is a linear refractive index, n.sub.2 I is a change portion of the refractive index by a light having an intensity I. The constant n.sub.2 is called as the nonlinear refractive index and has a relation with .chi..sup.(3) as shown below (in cgs unit system): ##EQU2##
The nonlinear refractive index change caused by excitation of electrons from the valence band into the conduction band in a semiconductor will be explained as follows.
When a light having a frequency higher than an energy of the band gap is irradiated into a semiconductor, a large number of electrons in the valence band are excited into the conduction band due to the light absorption in the semiconductor. However, the excitation of electrons from the valence band into the conduction band becomes difficult as the conduction band becomes filled with the electrons excited from the valence band (band filling effects).
The refractive index of the semiconductor is affected by the band filling effects, because the refractive index is dependent on the real component of the optical nonlinearity .chi..sup.(3), and the optical nonlinearity .chi..sup.(3) is affected by the band filling effects.
The transition time of electrons from the valence band into the conduction band due to the light absorption is as short as some 100 fs, so that the nonlinear refractive index is thought to appear almost the same time as the light is supplied to the semiconductor.
According to the conventional nonlinear optical device for controlling a signal light by a control light, however, there is a disadvantage in that the operation speed of the nonlinear optical device is limited, because changes of the nonlinear refractive index can not follow changes of the control light. In more detailed description, the carriers excited in the semiconductor do not disappear quickly after stopping of the control light supply. This phenomenon is represented by T.sub.1 (the longitudinal relaxation time or the re-combination time) in the formula (1). The longitudinal relaxation time T.sub.1 is equal to or over 10 ns, so that the optical nonlinearity .chi..sup.(3) cannot follow the change of the control light in frequencies higher than 0.1 GHz. The relaxation time is equal to or over 10.sup.-9 s even in a direct transition type semiconductor such as GaAs. This is not enough for applying to a very high speed operation.
Furthermore, the optical nonlinearity .chi..sup.(3) has a tendency to become small as the relaxation time becomes short, so that the performance of the nonlinear optical device may become poor, in other words, it takes more power for the control operation.
Generally, a nonlinear material which has a large optical nonlinearity .chi..sup.(3) has a large relaxation time. Considering one semiconductor material, the optical nonlinearity .chi..sup.(3) is generally large in the phenomenon of the refractive index change, and the relaxation time is also large. The detailed description about the phenomenon is shown by R. A. Fishered in "Optical Phase Conjugation, chapter 10, 1983".
Consequently, it is difficult to obtain a nonlinear optical device which operates at a high operation speed and a low control power.